The present invention relates to an alignment mark on a semiconductor wafer capable of obtaining high alignment accuracy and a method of forming the same.
A photolithographic process is necessary to manufacture IC semiconductor devices. This process includes an alignment step which properly positions an alignment mark on the surface of a semiconductor wafer with the corresponding alignment mark on the surface of a photo mask on which IC patterns are depicted.
Recently, the alignment step is automatically performed by using an alignment and exposure apparatus with a photoelectric edge detector. A typical conventional engraved alignment mark is shown in FIG. 1. The alignment mark 11, for example, is formed by removing an oxide layer 16 on a semiconductor wafer 14 in a form of a cross having a width of 6 .mu.m, and generally placed in a grid line 13 between semiconductor chip regions 15.
In the alignment step, the alignment and exposure apparatus places a photo mask (not shown) over the semiconductor wafer 14, roughly registers the alignment mark on the photo mask with the corresponding alignment mark 11 on the semiconductor wafer 14, scans the surface of a thick photo-resist layer (not shown) on the alignment mark 11 with light (ultraviolet rays) through the alignment mark of the photo mask in the direction of the arrow a, receives the two beams of light reflected from and near the edges 12 and 17 of the alignment mark 11 by its edge detector, calculates with the output signal from the edge detector the relative position between the alignment mark of the semiconductor wafer 14 and that of the photo mask, and adjusts the position between the wafer 14 and the photo mask according to the calculated output data.
In general, these steps are repeated several times until the alignment mark of the wafer 14 is coaxially symmetrically interposed in the middle of the alignment mark of the photo mask.
FIG. 2 shows a reflected light strength distribution on the surface of the alignment mark 11 obtained by the edge detector. In FIG. 2, for example, a thick line 32' shows a small amount of light reflected from the edge 12 shown in FIG. 1 and a thin line 32 a large amount of light reflected from the edge 17 shown in FIG. 1. FIG. 2 also shows double or triple interference fringes 34 or 35 which disturb accurate detecting operation of the edge detector.
In light scanning, the edge detector generates two peak signals as shown in FIG. 3. The negative or minus peaks 21 and 22 correspond to the edges 12 and 17 of the mark 11, respectively. The slopes 23 and 24 also show the levels of light reflected from and near the edge of the alignment mark on the photo mask.
The edge detector generates two pulses having the same amplitude in normal conditions. However, in case either edge (for example, edge 17) of the alignment mark 11 reflects enough incident light without attenuating due to the surface condition of a sloped thick photo-resist layer formed on the edge 17 of the alignment mark 11, the edge detector often generates a small pulse 22 as shown in FIG. 3. If two beams of reflected light are extremely different in light amount from each other, the edge detector generates wrong data to adjust the relative position between the photo mask and the semiconductor wafer 14. The difference between light levels received by the edge detector makes an alignment and exposure apparatus erroneously operate.
Therefore, with a conventional alignment mark, it has been difficult for an alignment and exposure apparatus to perform a high accurate alignment operation.